Recent enhancement in bonding and mounting techniques is associated with high-density packaging of a semiconductor device mounted in electronic equipment as well as elaboration of a package and high-density wiring of a printed wiring board, and electronics are continuously progressing. In a printed wiring board constituting such electronic equipment, both multilayering and fine wiring are proceeding. For realizing speeding-up of signal transmission speed required so as to increase the information processing speed, it is effective to decrease the dielectric constant of the material used. A polyphenylene ether is excellent in high-frequency characteristics (i.e., dielectric characteristics) such as dielectric constant and dielectric loss tangent, and therefore is suitable as a material for a printed wiring board of electronic equipment utilizing a high frequency band.
As a technique for enhancing heat resistance and dimensional stability of a polyphenylene ether that is a thermoplastic resin, Patent Document 1 describes a resin composition containing a polyphenylene ether and a triallyl isocyanurate (TAIC). As a technique for enhancing chemical resistance, Patent Document 2 describes a resin composition containing a maleated polyphenylene ether providing a reaction product of a polyphenylene ether and maleic anhydride, and TAIC. However, in the resin composition described in Patent Document 1 or 2, the melting point of the resin itself is high to make the viscosity during melting too high at an ordinary press-molding temperature, and the resin composition cannot be easily filled to form an inner-layer conductor pattern layer of a multilayer printed wiring board. Thus, there is a problem that multilayering of a wiring board is difficult.
For the purpose of improving such a problem in moldability, Patent Document 3 describes a polyphenylene ether resin composition using a polyphenylene ether with a low molecular weight and thereby ensuring that the molten resin has good fluidity and the moldability at an ordinary press-molding temperature is excellent to enable multilayering. However, decreasing the molecular weight of a polyphenylene ether incurs a problem that the heat resistance of the obtained laminate sheet decreases and a problem that due to increase in the number of terminal hydroxyl groups of the polyphenylene ether, the dielectric loss tangent becomes large. Accordingly, this technique is also not sufficient to be used for a printed wiring board.
For the purpose of improving these problems posed by the decrease in the molecular weight of a polyphenylene ether, Patent Documents 4 and 5 describe use of a low-molecular-weight •terminal-sealed polyphenylene ether where the terminal hydroxyl group of a low-molecular-weight polyphenylene ether is sealed with a reactive functional group. It is believed that by using this polyphenylene ether, a cured product prevented from reduction in the heat resistance or reduction in the dielectric constant and dielectric loss tangent while maintaining good moldability at the press-molding is obtained. Furthermore, Patent Document 4 also describes a method of using the low-molecular-weight•terminal-sealed polyphenylene by mixing it with a normal polyphenylene ether. In Reference Example 7 of Patent Document 4, a polyphenylene ether having a number average molecular weight of 14,000 and a terminal-ethenylbenzylated polyphenylene ether having a number average molecular weight of 2,500 are used in a mixing ratio of 5:70, and in Reference Example 8, a polyphenylene ether having a number average molecular weight of 14,000 and a terminal-ethenylbenzylated polyphenylene ether having a number average molecular weight of 2,500 are used in a mixing ratio of 50:60.
The method of using a polyphenylene ether having a normal molecular weight by mixing it with a polyphenylene ether reduced in the molecular weight is also described, for example, in Patent Documents 6 and 7. Patent Document 6 describes a technique where for the purpose of enhancing the heat resistance of a low-molecular-weight •terminal-functionalized polyphenylene ether, from 0.1 to 9.1% of a normal polyphenylene ether is blended to the low-molecular-weight•terminal-functionalized polyphenylene ether.
Patent Document 7 describes a resin composition where a polyphenylene ether having a normal molecular weight and being functionalized with an acyl group or an electrophilic group (intrinsic viscosity: 0.35 dl/g or more) and a non-functionalized polyphenylene ether reduced in the molecular weight (intrinsic viscosity: approximately from 0.15 to 0.35 dl/g) are mixed in a ratio of 40 to 55:60 to 45.